According to scientists at the Scripps Institution of Oceanography at the University of California San Diego, CO2 at Mauna Loa measured 399.5 ppm last week, the highest concentration since scientists there began measuring the gas in 1958. Researchers say the level compares to the Pliocene era, according to analysis of phytoplankton preserved in ocean sediment.

The world 4 million years ago was a few degrees warmer than now (and 10 degrees warmer at the poles). The oceans extended further inland. Tropical plants and animals reached northward.

For a variety of reasons, including manmade alterations to the planet, scientists say they’re uncertain whether current CO2 levels will return Earth to similar conditions. “The main lagging indicator is likely to be sea level just because it takes a long time to heat the ocean and a long time to melt ice,” said Richard Norris, a geologist at Scripps, on the organization's website.

So even after man stops warming the planet, the effects will continue.